1. Field of the Invention
The present invention relates to a laminated semiconductor ceramic capacitor with a grain boundary-insulated structure and a method for manufacturing the same, and more particularly to a ceramic capacitor which absorbs both low voltage noises and high frequency noises under normal operational conditions, and works as a varistor against invading high voltage pulses and high voltage static electricity, thereby protecting built-in semiconductors and electronic equipment from being damaged by abnormal voltages caused by noises, pulses, and static electricity generated within the electronic equipment.
2. Description of the Prior Art
In recent years, semiconductor devices such as ICs and LSIs have been widely used in electronic equipment for the purpose of realizing multifunctional applications of the equipment and making the equipment light, small, and handy. However, the use of many semiconductor devices has caused the decrease in noise resistance of the equipment. In order to maintain the noise resistance of the electronic equipment, by-pass capacitors such as a film capacitor, a laminated ceramic capacitor, and a semiconductor ceramic capacitor have been integrated into power source lines of various kinds of ICs and LSIs. These capacitors exhibit excellent performances in absorbing low voltage noises and high frequency noises. However, these capacitors themselves do not have a function of absorbing high voltage pulses and high voltage static electricity, so that the invasion of high voltage pulses and high voltage static electricity causes the malfunction of the equipment, and the break-down of semiconductors and/or capacitors.
As a new type capacitor, which has sufficient pulse resistance and excellent pulse absorptivity as well as good noise absorptivity and stability with respect to temperature and frequency changes, a semiconductor ceramic capacitor with a grain boundary-insulated structure in which a varistor function is provided to an SrTiO.sub.3 type semiconductor ceramic capacitor (hereinafter, this type of capacitor is referred to as a ceramic capacitor having a varistor function) has been developed. This kind of ceramic capacitor is disclosed in Japanese Laid-Open Patent Publication Nos. 57-27001 and 57-35303. Usually, this ceramic capacitor having a varistor function absorbs low voltage noises and high frequency noises, and when high voltage pulses and high voltage static electricity invade it, the capacitor works as a varistor, thereby protecting the electronic equipment and built-in semiconductors from abnormal voltages caused by noises, pulses, and static electricity generated in the electronic equipment. Thus, its applications have been increased in various fields.
Since electronic parts have been made lighter, smaller, more handy, and higher in their performance, there has been a strong demand for miniaturizing and achieving high performance of a ceramic capacitor having a varistor function. However, since a conventional ceramic capacitor having a varistor function is of a single-plate type, so that the effective electrode area becomes small when the capacitor is miniaturized, leading to a decrease in capacitance and inferior reliability. As a solution to the above-mentioned problems, it is considered that an electrode area be enlarged by laminating sheet-shaped electrodes.
In general, a capacitor which does not have a lamination structure is manufactured by a method comprising the steps of: sintering a ceramic sheet under a reducing atmosphere to make ceramics of the green sheet semiconductive; insulating a grain-boundary portion of the semiconductive ceramics by oxidizing the green sheet in air; and attaching an outer electrode to the resulting green sheet. However, when this method is applied to the above-mentioned laminated capacitor, the following problems arise. When the green sheet having an inner electrode on the surface thereof is laminated and sintered, cracks are generated or the inner electrode is oxidized so as to be insulated during the step of oxidizing because of the difference in coefficient of contraction between the inner electrode and the ceramics constituting the green sheet. Accordingly, it has been considered to be very difficult to manufacture a lamination-type capacitor having a varistor function (hereinafter, this type of capacitor is referred to as a laminated ceramic capacitor having a varistor function) by sintering a material of the ceramic capacitor having a varistor function and a material of the inner electrode at the same time, wherein the material of the ceramic capacitor includes the green sheets, a material of outer electrodes, etc. but which excludes the material of the inner electrode.
As a method for simultaneously sintering a material of the laminated ceramic capacitor having a varistor function and a material of the inner electrode, the following method for manufacturing a laminated ceramic capacitor having a varistor function has been developed and proposed, which employs processes disclosed in Japanese Laid-Open Patent Publication Nos. 54-53250 and 54-54250, comprising the steps of: printing a pattern corresponding to the inner electrode by using ceramic paste enriched in organic binder on the surface of the ceramic substrate; sintering the ceramic substrate to make the pattern of the inner electrode porous; and impregnating the porous pattern with electrically conductive metals under the appropriate pressure, or alternatively a step of forming a pattern of the inner electrode by a gilding or fusion method. However, these processes involve many production difficulties and they have not reached a practical level.
Japanese Laid-Open Patent Publication No. 59-215701 discloses a method comprising the steps of: forming a green sheet made of ceramic powder calcinated under a non-oxidizing atmosphere; printing a pattern of the inner electrode by using an electrically conductive paste mixed with a thermal diffusion material on the surface of the green sheet, the thermal diffusion material having the ability to form an electrically insulated layer in a grain boundary; and sintering the green sheet under the oxidizing atmosphere. Another method disclosed in Japanese Laid-Open Patent Publication No. 63-219115 comprises the steps of: forming a green sheet made of semiconductive ceramic powder as a main component, the main component being mixed with an oxidizing agent and/or a dispersing agent containing a glass component for the purpose of forming an insulated layer; alternately laminating the green sheets with a plurality of inner electrodes; and sintering the laminated sheets in air or under the oxidizing atmosphere. However, according to the above-mentioned two methods, sintering temperature is relatively low in the range of 1,000.degree. to 1,200.degree. C., so that the ceramics are not readily sintered and crystal grains do not come into contact with each other. The resulting ceramic capacitor which is not a completely sintered body entails shortcomings including: a relatively small electrical capacitance; a small value of voltage non-linear index (the non-liner index is a representative factor for showing charcteristics for a varistor); and instability of varistor voltage; and inferior reliability as a capacitor. Moreover, in Japanese Laid-Open Patent Publication No. 63-219115 involving the step of adding a glass material to the ceramic powder as an additive entails a problem in that a glass phase deposits in the crystal grain boundary, whereby the electrical characteristics tend to be degraded and the reliability of the capacitor becomes poor. Thus, this method has not reached a practical level.
Then, as described in Japanese Patent Application No. 1-36757, the present inventors have improved a method for producing a capacitor by using a composition of SrTiO.sub.3 containing an excess amount of Ti as a semiconductor component and MnO.sub.2 -SiO.sub.2 type compound as a base material; and have made it possible to develop a laminated ceramic capacitor with a varistor function which has inner electrodes essentially made of Au, Pt, Rh, Pd or Ni. Since Ni inner electrodes may cause the oxidation of Ni at relatively lower temperatures, thereby being insulated easily, a mixture of Ni and Pd, or a SiTiO.sub.3 containing a slightly excess amount of Ti has been proposed to use as an inner electrode material. However, even in this case, because Ni may be oxidized at a re-oxidation temperature of 1200.degree. C. or more, it is difficult to obtain a substantial resolution to the above-discussed disadvantages.
As a patent concerning the laminated capacitors having a varistor function, laminated voltage non-linear elements made of ceramic material such as ZnO, Fe.sub.2 O.sub.3, or TiO.sub.2 have been provided in Japanese Patent Publication No. 58-23921. This type of element has a very small capacitance. Therefore, although it exhibits excellent performance with respect to pulses having a relatively high voltage and absorption of static electricity, it exhibits little effect with respect to noises with a voltage below the varistor voltage or high frequency noises.
As described above, in the laminated ceramic capacitor having a varistor function using Ni as inner electrodes, Ni is oxidized at relatively low temperatures. Therefore, depending on the production method, Ni is oxidized and inner electrodes are insulated, whereby electrical characteristics cannot be obtained. Accordingly, it is expected that novel inner electrode compositions which prevent Ni from being oxidized and a method for manufacturing laminated ceramic capacitors having a varistor function using such Ni inner electrodes are being developed.